JP2004-119434A discloses a method for evaluating fatigue of Sn—Ag system solder joint portion. According to the evaluation method disclosed, the focus is on Sn phase and Ag3Sn phase that exists independently from the Sn phase. Then, a dimension d of the Ag3Sn phase and the Sn phase is measured. A phase growth evaluation parameter S is obtained by calculating the fourth power of the measured dimension d. The Sn phase and the Ag3Sn phase are enlarged while being exposed to thermal cycles and therefore the dimension d increases as a whole. A relationship between a thermal fatigue crack initiation cycle Ni corresponding to thermal cycles with which the fatigue crack is generated in the solder joint portion, and the phase growth evaluation parameter S is then determined. That is, the thermal fatigue crack initiation cycle Ni and a phase growth evaluation parameter Si at that time have a following relationship therebetween: Si=A*Niα wherein A and α are constants determined depending on a type of solder. A thermal fatigue life of the solder joint portion caused by application of the thermal cycles, i.e., a lifetime before the crack is initiated in the solder joint portion, is determined on the basis of a change rate ΔS of an increase of the phase growth evaluation parameter Si.
JP2004-85397A discloses a method for estimating a lifespan of a solder joint portion using a simulation model. According to the method disclosed, a simulation model in which a crack is not generated is prepared and used for obtaining inelastic strain occurring in the solder joint portion at a time temperature cycles are applied to the solder joint portion. Then, a crack initiation cycle is acquired on the basis of the inelastic strain obtained. On the basis of a relationship between a crack propagation speed obtained by a crack propagation test on solder material and a fracture mechanics parameter, and a relationship between the fracture mechanics parameter obtained by using the simulation model and a crack length, the crack propagation speed is obtained as a function of the crack length to thereby acquire a crack propagation lifespan that is a lifetime before the crack reaches a predetermined length.
However, according to the method for evaluating fatigue of a solder joint portion disclosed in JP2004-119434A, only the thermal fatigue crack initiation cycle is predicted, which may be insufficient for the prediction of the lifespan of the solder joint portion. In addition, the method for estimating a lifespan of a solder joint portion disclosed in JP2004-85397A does not include an actual test and thus durability is insufficient. In the related industry, it is desired to develop a method for evaluating a solder joint portion that can further effectively evaluate the lifespan of the solder joint portion.
A need thus exists for a method for evaluating a solder joint portion which is not susceptible to the drawback mentioned above.